The present invention relates to continuous caster molds having an opposed pair of broad walls and an opposed pair of adjustably spaced narrow walls, and particularly to such a continuous caster mold and method of utilizing the mold for the production of relatively thin cast slabs.
In the continuous casting of steel, it is known to utilize molds having a pair of opposed broad walls and a pair of opposed narrow walls in which at least one of the narrow walls is adjustable for changing the width of the strand during the actual casting operation. Normally, the thickness of the strand being cast on currently existing casters is within the range of six to twelve inches (15 to 30 cm). For slabs of this thickness, the mold walls are essentially rectangular so that the broad walls are essentially equally spaced from each other at all elevations of the mold. Recently, work has been done to reduce the thickness of the slab being cast in order to bypass the roughing stands in a hot-strip mill. By casting a thinner slab, less reheating and, perhaps, no reheating may be required prior to charging into the strip mill. Thus, a thinner slab can eliminate the need for reheat furnaces and roughing stands as well as intermediate conditioning operations, thereby lowering the capital and production costs of making steel strip while increasing the yield.
The preferred thickness of a thin slab is 2 inches (5 cm) or less. Conventional continuous casters are generally adapted to cast slabs on the order of six to twelve inches (15 to 30 cm) in thickness, i.e., three to four times the thickness of a preferred thin slab. In downsizing a conventionally designed continuous casting mold to cast a thin slab, a difficulty is encountered in that molten metal introduced between the closely spaced, water cooled walls tends to solidify very quickly near the introduction point rather than uniformly. No practical means have been developed for introducing molten metal uniformly into a thin slab caster.
West German Patent No. 887,990, issued Aug. 27, 1953 to Rossi, describes a thin caster mold having a funnel-like upper portion adapted to receive liquid metal. The mold of 887,990 is not adjustable.
U.S. Pat. No. 4,635,702, issued Jan. 13, 1987 in the name of Kolakowski, et al., discloses a thin caster mold having straight-sided narrow walls adjustable to different slab widths. The broad walls of this mold have a funnel shape in their upper central portion adapted to receive liquid metal. A disadvantage of this mold is that the spacing to which the narrow walls may be adjusted is limited to only the length of part of the straight side portions of the outer ends of the broad walls and not into the funnel-shaped central portion thereof. A further disadvantage of this mold relates to the narrow sidewall adjustment mechanism. The narrow walls of this mold are extremely narrow, being on the order of 2 (5 cm) inches wide. These walls are movable in and out to vary the width of the slab being cast. The tilt angle of the narrow walls must also be adjustable since the optimum tilt angle is a function primarily of the slab width. Other factors, such as the casting speed and tundish temperature, affect the preferred tilt angle at any given slab width. Due to the extremely narrow width of the side walls of Kolakowski et al., the width of the side wall adjustment mechanism is also limited. For use in adjusting the mold width and the narrow wall tilt angle, Kolakowski et al. employs a pair of thin threaded spindles attached to each narrow mold wall. Such spindles suffer from the lack of strength and vibration resistance. A further disadvantage of this design is that for each side of the mold, the narrow wall is tiltable only in its entirety, i.e., the top half and the bottom half are not independently adjustable.
It is known in heavier thickness caster molds to have the narrow walls tiltable about the central or mid-elevation axis of the narrow walls, for example, as shown in U.S. Pat. Nos. 3,926,244, Meier, et al; 4,505,321, Zeller; and 3,375,865, Boichenko, et al. Also, curved inner and outer surfaces on caster mold narrow walls are disclosed in U.S. Pat. No. 4,023,612, Jackson.
Moreover, these above-described prior molds are usually adapted to allow slabs of differing widths to be produced by movement of the narrow end walls. Such width changes are however, normally made only after casting is stopped or interrupted, thereby slowing the casting process and creating "transition castings", comprising a first portion of the newly or most recently cast slab and a second portion of the output slab which was cast just before casting was interrupted. This transition casting is usually disposed of as waste since it does not have uniform strength and other desirable properties. This casting interruption methodology is described in U.S. Pat. No. 4,245,692, issued on Jan. 20, 1981 in the name of Hargassner et al; and Japanese Patent Number 59-189047 which issued on Oct. 26, 1984.
It is a primary object of this invention to provide a continuous caster mold and method for casting relatively thin strands, the mold having narrow walls and the mold and method overcoming the disadvantages of the prior art.